1. Field of the Invention
In casting and molding processes in general, front surface and back surface dies are used to impart to the plastic material the optical surfaces appropriate for the desired ophthalmic correction. In casting processes in general, the dies are secured together by a gasket or the like at a desired spacing, and a liquid casting material is introduced in the casting cavity defined thereby, and allowed to cure and harden into a lens. A critical factor is the gasket, and that a large plurality of gaskets are required to accommodate and seal with the large plurality of differing optical surfaces of the various dies while also defining the desired die spacing (and lens thickness). Thus a large capital investment is required in gaskets and gasket tooling, as well as in the optical surface forming dies used in the casting process.
2. Background of the Art
In molding processes in general, more rugged dies are employed to compress a moldable material in a closed molding cavity, impressing the optical surface configurations on the material as the material is hardened by thermal or chemical means. Generally speaking, an excess of molding material must be introduced into the cavity to allow for compressive die movement, and means must be provided to vent the excess material from the cavity and accommodate the sprues formed thereby. Also, as in the casting process, the dies must be urged together to the appropriate spacing to create the desired lens thickness. The amount of molding material, the venting of excess material, and the final thickness of the lens are interrelated factors that require precise process control to produce lenses of high quality for ophthalmic use.
U.S. Pat. No. 4,839,110 describes a process for molding or casting ophthalmic lenses in which the apparatus includes a plurality of front surface and back surface dies having differing optical forming surfaces and identical peripheral edge configurations. In the compression molding process, any pair of front surface and back surface dies are received in sliding, sealing fashion in the bore of the molding apparatus. At least one of the pair of dies is provided with at least one ,ate channel formed in the peripheral edge thereof parallel to the axis of the bore and extending from the optical forming surface to a point adjacent to the opposite, external die surface. The gate channel comprises a vent through which excess molding compound is discharged from the molding cavity as the dies are urged together. As the gated die is fully inserted in the bore, the gate channel opening to the exterior is sealed by the bore surface, thus terminating the discharge effect and retaining a predetermined amount of molding material in the molding cavity. The molding material is then solidified by thermal or chemical reaction to form the finished lens. Thus the invention provides intrinsic control of the thickness of the molded part, while also providing positive venting of excess molding material. In the casting, process, the dies are partially inserted in the bore of a casting in sliding, sealing fashion, and the gate channel serves as an injection channel to introduce the casting compound into the casting cavity. The dies are urged together in the bore, the gate channel first serving to vent excess casting compound and gas from the casting cavity. Full die insertion in the bore causes the gate channel opening, to the exterior to be sealed by the bore wall, thus terminating the discharge effect and retaining a predetermined amount of casting material in the casting cavity. The material is then cured or hardened by thermal or chemical reaction to form the finished lens. Thus the invention also provides intrinsic control of the thickness of the cast part, while also permitting easy filling of the casting cavity and venting of excess material and gas from the vent cavity.
U.S. Pat. No. 5,288,221 describes an alternative method of preparing an ophthalmic lens comprising a process wherein a premanufactured polymeric lens or lens wafer is employed as one of two molds required to produce the finished lens. A conventional glass, plastic, ceramic, or metal mold is employed as the second mold, a polymerizable lens material is injected between said two molds, and the relative positions of said two molds are adjusted as required by the prescription to produce the finished lens. The polymerizable lens material and the premanufactured lens or lens wafer adhere to one another through a combination of mechanical and chemical bonding. The joined lens material and the lens wafer are then removed from the glass, plastic, ceramic, or metal mold. In this manner, one of the two molds used to make the lens is incorporated into the lens and becomes an integral part thereof. The respective orientation of the two molds is prepared by a pair of indexing means. The indexing means control the position of one of the two molds while the other mold is held in a fixed position. A first indexing means controls the rotational relationship between the molds and a second indexing means controls the spatial relationship between them. An indicator associated with the first indexing means indicates the angular position of the movable mold on a real time basis and an indicator associated with the second indexing means similarly indicates the instantaneous spatial relationship between the two molds so that the shape and thickness of the polymerizable material between the molds is continuously known; this enables the eye care professional to adjust the indexing means as needed.
Although the above-identified semi-finished lens manufacturing processes are of the types of manufacturing processes that have been commercially successful over the years, there are definite limitations to their utility. These manufacturing processes have been used to manufacture monofocal lenses for a number of reasons. One reason is that processes require significant modification to exchange either one of the lens mold surfaces. This is important because any multifocal lens must have a unique combination of both of the corrective functions of the multifocal lens. Both the cylinder and the diopter of the product must be matched to a specific prescription. As there are many combinations of prescriptions in the public, it has been considered too time consuming to manufacture a full range of prescriptions, particularly with the time delays in changing mold elements within the molding process. Additionally, the molding processes, with their mated diameter mold faces have not been able to control edge thicknesses easily and/or tend to place stress in the ends of the manufactured lenses in injection molding processes which lead to potential physical stresses in the final lens that can result in stress to the plastic under extreme ambient conditions. This is in spite of the fact that numerous alternative processes have been developed for injection molding.
Illustrative of these problems in the context of optical disk molding is Bartholdsten et al (U.S. Pat. No. 4,409,169), which teaches the need for a slow (up to 3 seconds), low-pressure injection of an oversized shot into a partially-open (air gap) mold parting line, then providing for deliberate melt cooling and viscosity-thickening, followed by a short pressing stroke (typically ⅕ to {fraction (1/10)} the disk""s thickness, or 0.005-0.010 inch) which initially squeezes out of the reduced mold cavity volume the partially-cooled and viscous excess plastic, then as the pressing continues to the fully-closed parting line position (zero clearance), this radially-extruded overflow is pinched off and full clamping force is thereafter maintained for shrinkage compensation and to assure no prerelease.
Another clamp-induced disk coining, process is disclosed in Matsuda et al (U.S. Pat. Nos. 4,442,061 and 4,519,763) wherein, into a slightly opened moldset, a melt is injected and cooled until fully solidified, then reheated till uniformly above the plastic""s melt temperature, at which point clamp-actuated compressive stroke is conventionally delivered and maintained through this second cooling cycle. The energy efficiency and total cycle time of such a process are highly questionable.
Another type of molding process (termed an xe2x80x9cauxiliary componentxe2x80x9d process for the sake of discussion) includes the use of auxiliary springs, cylinders or the like to apply a compressive force to the opposing optical surfaces and which compressive force means are commonly internal to the mold itself or as peripheral apparatus thereto. The primary difference between xe2x80x9cauxiliary componentxe2x80x9d molding and clamp-end injection/compression, is that mold compression is provided by a stroke-producing element inherent to some injection molding machines (examples of same are the ejector or movable platen driving mechanisms such as the main clamp) in the latter whereas mold compression is provided by auxiliary springs or hydraulic cylinders, for example, in auxiliary component molding. Furthermore, clamp-end injection/compression motions tend to be inherently sequenced through and coordinated by the molding machines process control system, whereas auxiliary component compression tends to be controlled (if not self-action, like springs) separately by timers, etc., not supplied with the standard machine.
Early thermoplastic lens molding by these types of processes employed simple spring-loaded, movable optical dies within the moldset (Johnson, U.S. Pat. No. 2,443,286). Such apparatus created a variable-volume lens mold cavity thereby, but relied upon high internal polymer melt pressure to spread the movable dies against the resisting spring pressure. In order to apply sufficiently great compressive forces upon the solidifying mold contents, these spring forces were great. However, the greater the spring force, the greater the injection pressure that must be used to compress the springs during variable cavity fill. The greater the injection pressure required, the greater the degree of molded-in stresses and optically unsatisfactory briefringence. Of course, the greater the optical power for the molded lens, the greater the dissimilarity between the front and back curves and thus the greater the cross-sectional thickness variation. Also, the larger the lens diameter or the thinner the nominal lens thickness, the worse the center-to-edge thickness variations and the higher the degree of difficulty in preventing differential thermal shrinkage and resulting optical distortion to the finished lens. Therefore, processes of this type were generally limited in practice to production only of weakly powered lenses with minimal diameter and minimal thickness variations.
A subsequent auxiliary component process is represented by Weber (U.S. Pat. Nos. 4,008,031 and 4,091,057). This process uses a variable-volume lens mold cavity. Weber teaches a variable-volume cavity formed by injection-melt, pressure-induced rearward deflection (in one embodiment) of at least one movable male or female die, which after a certain interval is followed by forward displacement resulting in compression under the driving force of an auxiliary hydraulic cylinder mounted in one-to-one relationship with this movable die. Flow ports are provided through which excess increasingly-viscous, partially-cooled injected polymer melt is forcibly extruded from the lens cavity under the compressive forces. In operation, Weber teaches a mold fill of approximately 10 seconds. Also, as with conventional clamp-induced coining, Weber relies upon a preset amount of time to elapse between completion of injection fill and commencing compressive pressure. Accordingly, problems related to either premature compression (inadequate solidification) or excessively delayed compression (late solidification) could be encountered.
Still another similar process is represented by Laliberte (U.S. Pat. No. 4,364,878). Laliberte includes a movable die coupled to an auxiliary hydraulic cylinder. After the mold is closed under clamp pressure, the mating die parts are spread apart. A precise volumetrically-metered shot size of predetermined quantity just adequate to fill the fully-compressed mold-cavity-and-runner system is then injected. This control of shot size allows compression free of partly-solidified melt displacement out of the mold cavity through an overflow port (as taught by Weber), permitting therefore greater control of nominal lens thickness and so eliminating material scrap waste and trimming operations (as required with Weber, for example).
The technical problems in the manufacture of ophthalmic lenses are quite significant. Prescription lenses must deal with the inherent differential-shrinkage caused by very thick and thin cross-sections within the same lens. Also, minus-powered lenses (having relatively flat front convex curve and relatively steeper back concave curve) must avoid melt knitlines or weldlines associated with edge gating Prescription lenses have, compared to disks, much lower aspect ratios (about 35:1) and lower projected area to clamp up (7 square inches for a typical lens vs 17.7 square inches for a typical optical disk). Also, an optical disk is formed between virtually identical planar mold cavity surfaces.
Compressive forces for such in-mold xe2x80x9cauxiliary componentxe2x80x9d approaches are much less than are available through clamp-actuated coining, and this limitation is particularly troublesome for disks because of their projected area and the necessity for intimate contact between the melt and the stamper.
U.S. Pat. No. 4,828,769 discloses a method for injection molding articles, especially ophthalmic lens blanks, which method comprises forming a closed mold cavity for receiving plasticized resin without introducing significant back pressure therein, injecting into the closed mold cavity a mass of plasticized resin slightly larger than the mass of the article to be formed, applying a main clamp force of the injection molding equipment to reduce the volume of the closed mold cavity, thereby redistributing the resin contained within the cavity, and maintaining the applied main clamp force, thereby compressing the resin at least until the resin within the closed mold cavity solidifies. This one-step method is described as at least addressing some of the problems encountered in prior art injection molding processes when they had been attempted for use with ophthalmic lenses. These types of manufacturing processes have been used exclusively for the preparation of monofocal lenses or for separate lens elements (e.g., front and rear lens elements) that are bonded together to form multifocal ophthalmic lenses.
Composite eyeglass lenses have been formed by bonding together front and rear lenses, as suggested in U.S. Pat. No. 2,618,200. A device and method for accomplishing this has been suggested in U.S. Pat. No. 4,927,480. Generally, the bonding process involves placing a curable adhesive on the concave interface surface of the front lens; pressing the convex interface surface of the rear lens against the adhesive in the front lens, to spread the adhesive throughout the space between the two lenses; and curing the adhesive to bond the lenses together, forming a composite lens, which is then trimmed to fit within an eyeglass frame.
Even after individual lens blanks of good optical properties have been manufactured, it is equally important to form them into ophthalmic lenses for use by the customer. Segmented and progressive ophthalmic lenses must also be capable of construction from these lens elements. For example, U.S. Pat. Nos. 4.883,548; 4,867,553; and 4,645,317 show the formation of laminated ophthalmic lenses from at least two separate lens elements which are selected from a reserve and then associated to match a particular description. The at least two lens elements are adhesively secured together, with a photosetting resin and photoinitiator suggested for the process (e.g., U.S. Pat. Nos. 4,883,548 and 4,867,553).
Especially when the desired composite lens includes a cylindrical component that must be properly oriented to correct for astigmatism and a bifocal or progressive focal region that must be properly positioned for reading purposes, the existing methods and equipment have fallen short of the desired optical accuracy. Existing laminating equipment, for example, does not readily accommodate eccentric positioning and bonding of the front and rear lenses, which can be necessary in some cases. Also, existing methods and equipment have been inconvenient to operate and have put the desirable accuracies beyond practical reach for some composite eyeglass lenses, U.S. Pat. No. 5,433,810 describes lamination or bonding, together of front and rear lenses to form a composite eyeglass lens to address these perceived problems
Although the use of separate laminable lens elements has proven to be of significant commercial advantage and has achieved acceptance with opticians and users, there would still be advantages if a finished or semifinished multifocal lens could be manufactured in a single procedure. This has not been accomplished with polymeric lenses, especially with polycarbonate lenses by either injection molding processes or casting processes especially where thinner (and therefore lighter weight) lenses are desired. Another aspect of the technical reasons why molding and casting have been unable to provide multifocal lenses with moderate to thin lens thickness dimensions (e.g., 0.5 to less than 2.0 mm edges and 1.0 to 3.5 center thickness) is the effect of the geometry of the lens, When molding plus power ophthalmic lenses, typically approximately equal dimension 75-85 mm bowl diameters are used for the front and the rear curves. The larger bowl diameter directly results in a thicker lens as a result of the geometry of the curves of the two bowls. This results in a heavier (because it is thicker) and a less attractive lens (because of the greater visibility of the edge of the lens, even with polishing).
It would be desirable to provide an effective process for the manufacture of finished or semifinished multifocal ophthalmic lenses with a single lens manufacturing step.
A process for the manufacture of multifocal ophthalmic lenses is provided for both injection molding and casting methodologies. The use of two opposed tools (mold faces) with relatively centered faces, but with significantly differing diameters can be used to provide multifocal ophthalmic lenses. The process is surprisingly capable of providing multifocal lenses with good control of the thickness of the lenses, with relatively thin center thicknesses (e.g., less than 3.5 mm) and relatively thin edge thicknesses (e.g., 0.5 to 2.0 mm). The tool faces (the molds for the front and rear surfaces) may be readily replaced within the mold to enable manufacture of a wide range of lens prescriptions with minimum down time.